Gel-type copolymer bead and ion-exchange resins made therefrom

ABSTRACT

Gel-type copolymer beads are described which are useful as ion exchange resins such as those used in the treatment of power plant condensate water. The subject copolymer beads possess an interpenetrating polymer network of at least two polymer components including: (i) a first polymer component derived from a first monomer mixture comprising a first monovinylidene monomer having a styrenic content less than about 50 molar percent and (ii) a second polymer component derived from a second monomer mixture comprising: a second monovinylidene monomer having a styrenic content greater than about 50 molar percent and a second crosslinking agent. The ratio of the molar percent crosslinking agent of the first polymer component to the second polymer component is less than about 0.7. Methods for making the subject beads and methods for their use as ion exchange resins for treating aqueous solutions are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/111,481, filed Dec. 9, 1998.

BACKGROUND OF THE INVENTION

Regular treatment of the condensate water of power plants is necessaryfor preventing corrosion and maintaining efficient water flow rates.Condensate water commonly contains dissolved and suspended materials,especially oxidized iron commonly referred to as “crud”. If notmaintained at low levels, crud can build up within the steam loop of thepower plant causing pressure build up, reduced flow rates, reduced plantefficiency and expensive repairs. Proper crud removal is particularlyimportant in boiling water nuclear reactors (BWRs) where crud can becomeradioactive as a result of passing through the “hot” side of the steamloop.

U.S. Pat. No. 4,975,201 (Re. 34,112), reissued Oct. 27, 1992(incorporated herein by reference), discloses processes for treatingpower plant condensate water by contacting the water with a mixed bedion exchange resin comprising cation and anion microporous copolymerbeads. These beads include an interpenetrating network of polymercomponents made by way of known suspension “seeded” polymerizationmethodologies. As described, the copolymer beads may be made by avariety of seeded polymerization techniques including insitu-type singleand second stage processes. These techniques typically include formingpolymeric seed particles (e.g., a first polymer component), suspendingthe seeds in a suitable suspension medium and continuously adding(“con-add”) a polymerizable monomer (e.g., a second polymer component)thereto, thereby forming an interpenetrating polymer network. In onealternative approach, the seeds are imbibed with a monomer mixture(e.g., a third polymer component) which is substantially polymerizedprior to the subsequent addition of the second polymer component notedabove. As indicated, suspension polymerization methodologies are wellknown in the art, see for example U.S. Pat. No. 4,564,644, which isincorporated herein by reference. These resins have been found to have ahigh capacity for removing crud from condensate water in BWR nuclearpower plants.

One drawback with current cation exchange resins is that they degradewith time to release a variety of sulfonated organic compounds. Themechanism for this resin degradation is believed to be due to anoxidative attack at the benzylic carbon groups of the resins. Subsequentchemical reactions cause the breakage of the copolymer chains. When twoor more breakages occur between crosslinked groups, sulfonated organiccompounds, i.e., “leachables”, are formed which can diffuse out of theresin and into the surrounding water. The molecular weight of theseleachables is typically between 150 and 100,000 daltons. Under the highoperating temperatures of the steam loop, these leachables cansubsequently desulfonate and release highly corrosive inorganic sulfateinto the water.

Anion exchange resins are effective at removing cation exchange resinleachables from solution so long as the total amount of leachables arelow and so long as the molecular weight of the leachables is relativelylow, i.e., below 10,000 (more preferably below about 5,000, and mostpreferably below about 1,000). Higher molecular weight leachables fromthe cation exchange resin are not effectively removed from solution byan anion exchange resin. As such, these leachables remain in the processwater as an impurity. Additionally, high molecular weight leachablesadsorbed by an anion exchange resin can lead to decreased kineticperformance of the anion exchange resin. Thus, it is desirable to reducethe average molecular weight of the cation exchange resin leachableswhile minimizing the total amount of leachables released into theprocess stream.

One way to reduce the total amount of leachables released by the cationexchange resin is to incorporate an antioxidant into the resin asdescribed in U.S. Pat. No. 4,973,607; however, improved cation exchangeresins are sought which release lower molecular weight leachables, andwhich release lower amounts of total leachables even without theaddition of such antioxidants. It is further desired to provide cationexchange resins which provide improved crud removal properties.

Another approach to reducing leachables from cation exchange resins isto increase the extent of crosslinking of each polymer component usedtherein. Although the use of increased crosslinking does not reduceoxidative attack of benzylic carbon atoms, it does reduce the likelihoodthat any given cleavage of a benzylic bond will lead to a leachablespecies. Unfortunately, with conventional exchange resins the degree ofcrosslinking and capacity for crud removal are somewhat inverselyrelated. Thus, as the extent of crosslinking is increased to reduceleachables, capacity for crud removal is compromised.

SUMMARY OF THE INVENTION

The present invention is a gel-type copolymer bead and method for makingthe same in which the subject copolymer beads have an interpenetratingpolymer network of at least two polymer components including:

a first polymer component derived from a first monomer mixturecomprising: a first monovinylidene monomer having a styrenic contentless than about 50 molar percent;

a second polymer component derived from a second monomer mixturecomprising: a second monovinylidene monomer having a styrenic contentgreater than about 50 molar percent and a second crosslinking agent; and

wherein the ratio of the molar percent crosslinking agent of the firstpolymer component to the second polymer component is less than about0.7.

An object of the present invention is to provide gel-type copolymerbeads which are useful as ion exchange resins and which have particularutility in the treatment of power plant condensate water. The subjectbeads provide ion exchange resins which are more effective at removingcolloidal iron from condensate water, release fewer total leachables,and/or release leachables of a lower molecular weight.

A further object of the present invention is to provide methods formaking the subject copolymer beads and methods for their use in treatingaqueous solutions.

DETAILED DESCRIPTION OF THE INVENTION

As indicated previously, the mechanism for cation exchange resindegradation is believed to be due to oxidative attack at the benzyliccarbon groups of the resins. Subsequent chemical reactions cause thebreakage of the copolymer chains leading to “leachables” (i.e., cleavedportions of the crosslinked resin) which diffuse out of the resin andinto the surrounding water. The present inventors have discovered thatby selectively reducing the styrenic content of the polymer componentsmost susceptible to oxidative attack, (i.e., those having relative lowermolar percent crosslinking agent), one can significantly reduce theaverage molecular weight and/or the total quantity of leachables. Forexample, conventional seeded sulfonated resins useful in treating powerplant condensate water comprise a seed component consisting of lightlycrosslinked polystyrene, along with additional polymer components,(e.g., imbibed and/or continuously added) consisting of highercrosslinked polystyrene. By replacing at least 50 molar percent of thestyrene content in the polymer component having the lowest degree ofcrosslinking (e.g., the seed component) with a non-styrenic monomer(e.g., acrylate, methacrylate, butadiene, ethylene, propylene,acrylonitrile, vinylidene chloride and vinyl chloride), one can producea strong, high capacity resin having lower total leachables and/orleachables of a significantly reduced average molecular weight and/orleachables containing a lower sulfur content while simultaneouslymaintaining and even increasing the crud removal properties of theresin.

Pursuant to the present invention, the polymer component(s) thatpreferably have styrenic contents below about 50 molar percent (and morepreferably below about 10 molar percent) are determined by the ratiobetween the molar percent crosslinking agent of any two polymercomponents. More specifically, if the ratio between the molar percentcrosslinking agent of any two polymer components is less than about 0.7(preferably less than about 0.4 and more preferably less than about0.1), then the polymer component having the lower molar percentage ofcrosslinking agent preferably has a monovinylidene monomer with astyrenic content of less than about 50 molar percent. When determiningthe aforementioned ratio, the polymer component having the lesser molarpercent crosslinking agent is the numerator and the polymer componenthaving the relative greater molar percent crosslinking agent is thedenominator. (In some embodiments of the present invention, a polymercomponent may include no crosslinking agent).

Although not required, the polymer component having the absolute lowestmolar quantity of crosslinking agent preferably has a styrenic contentof less than 50 molar percent. Depending upon the specific application,operating conditions and relative weight ratios of each polymercomponent, it may be desirable to have all polymer components whichsatisfy the aforementioned ratio to have less than about 50 molarpercent styrenic content; however, for purposes of this invention, onlyone of the polymer components which satisfy the aforementioned criterianeed have a styrenic content less than 50 molar percent.

As used herein, the term “polymer component” refers to the polymericmaterial resulting from a distinct polymerization step. For example, theresins of the present invention are preferably “seeded” resins. Thus,the formation of the seed particles constitutes a distinct polymercomponent. Similarly, the process step of imbibing and polymerizing amonomer mixture into the seed constitutes yet another polymer component.If used, the subsequent continuous addition of a monomer mixturecommonly used to “grow up” the seed also constitutes a distinct polymercomponent. Except as specifically described herein, the constituents ofeach polymer component may be the same or different. Moreover, themonomer mixture used during a polymerization step need not behomogeneous; that is, the ratio and type of monomers may be varied. Theterm “polymer component” is not intended to mean that the resultingresin have any particular morphology other than an interpenetratingpolymer network; however, the present resins typically have a“core-shell” type structure as is described in U.S. Pat. No. Re 34,112.The polymer components of the present invention preferably includepolymeric material which contributes more than about 5 weight percent ofthe final polymerized copolymer bead. Typically, the resins of thepresent invention comprise two or three polymer components, i.e., a seedcomponent, imbibe component, and/or a continuous addition component.Those skilled in the art will appreciate that different or additionalcombinations of polymer components may be used, e.g., multiple con-addcomponents may be utilized. The first, second, third, etc. polymercomponents do not necessarily correspond to an order of addition. Thatis, the “first polymer component” does not necessarily correspond to thepolymer component which is first polymerized, e.g., a seed particle. Theterms “first” and “second” are only used to distinguish one componentfrom another, not to designate an order of addition.

The term “monovinylidene monomer” is intended to include homogeneousmonomer mixtures and mixtures of different types of monomers, e.g.,styrene and isobornylmethacrylate. Similarly, the term “crosslinkingagent”, “crosslinker” and “crosslinking monomer” are intended to includeboth a single species of crosslinking agent along with combinations ofdifferent types of crosslinking agents. The term “styrenic content”refers to quantity of monovinylidene monomer units of styrene and/orsubstituted styrene. Substituted styrene includes substituents ofeither/or both the vinylidene group and benzyl group of styrene, e.g.,vinyl naphthalene, alpha alkyl substituted styrene (e.g., alpha methylstyrene) alkylene-substituted styrene (particularlymonoalkyl-substituted styrene such as vinyltoluene andethylvinylbenzene) and halo-substituted styrene, such as bromo- orchlorostyrene and vinylbenzylchloride.

The copolymer beads of the present invention are preferably prepared bysuspension polymerization of a finely divided organic phase comprisingmonovinylidene monomers, polyvinylidene monomers such as divinylbenzene,a free-radical initiator and, optionally, a phase-separating diluent.The copolymer beads produced are microporous, i.e., gel-type incharacter as contrasted with macroporous wherein a phase-separatingdiluent is employed. The term “macroporous” as commonly used in the artmeans that the copolymer has both macropores and mesopores. The terms“microporous”, “gel” and “macroporous” are well-known in the art andgenerally describe the nature of the copolymer bead porosity.Microporous or gel-type copolymer beads have pore sizes on the order ofless than about 20 Angstroms (Å), while macroporous copolymer beads haveboth mesopores of from about 20 Å to about 500 Å and macropores ofgreater than about 500 Å. Gel and macroporous copolymer beads, as wellas their preparation, are further discussed in U.S. Pat. No. 4,256,840.

As indicated, the resins of the present invention are preferably made byway of a seeded polymerization. Seeded polymerizations, also known ascontinuous or semi-continuous staged polymerizations, are generallydescribed in U.S. Pat. Nos. 4,419,245 and 4,564,644, the relevantteachings of which are incorporated herein by reference. A seededpolymerization process typically adds monomers in two or moreincrements, each increment comprising at least about 5 percent, andpreferably at least about 10 percent of the weight of the resultingcopolymer beads. Each increment is followed by complete or substantialpolymerization of the monomers therein before adding a subsequentincrement.

A seeded polymerization is advantageously conducted as a suspensionpolymerization wherein monomers, or mixtures of monomers and seedparticles, are dispersed and polymerized within a continuous suspendingmedium. In such a process, staged polymerization is readily accomplishedby forming an initial suspension of monomers, wholly or partiallypolymerizing the monomers to form seed particles, and subsequentlyadding remaining monomers in one or more increments. Each increment maybe added at once or continuously. Due to the insolubility ofethylenically unsaturated monomers in the suspending medium and theirsolubility within the seed particles, the monomers are imbibed by theseed particles and polymerized therein. Multistaged polymerizationtechniques can vary in the amount and type of monomers employed for eachstage, as well as the polymerizing conditions employed.

The seed particles employed may be prepared by known suspensionpolymerization techniques. In general, the seed particles may beprepared by forming a suspension of a first monomer mixture in anagitated, continuous suspending medium, as described by F. Helfferich inIon Exchange, (McGraw-Hill 1962) at pp. 35-36. The first monomer mixturecomprises at least one first monovinylidene monomer, a firstcrosslinking monomer, and an effective amount of a first free-radicalinitiator. The suspending medium may contain one or more suspendingagents commonly employed in the art. Polymerization is initiated byheating the suspension to a temperature of generally from about 50° C.to about 90° C. The suspension is maintained at such temperature untilreaching a desired degree of conversion of monomer to copolymer. Othersuitable polymerization methods are described in U.S. Pat. Nos.4,444,961; 4,623,706; and 4,666,673.

The monomers employed herein are addition polymerizable ethylenicallyunsaturated compounds. Such monomers are well-known and reference ismade to Polymer Processes, edited by Calvin E. Schildknecht, publishedin 1956 by Interscience Publishers, Inc., New York, Chapter III,“Polymerization in Suspension” at pp. 69-109, for purposes ofillustration. In Table II, on pp. 78-81, of Schildknecht are listeddiverse kinds of monomers which are suitable in practicing thisinvention. Of such ethylenically unsaturated monomers, of particularinterest are water-insoluble monovinylidene monomers including themonovinylidene aromatics such as styrene and substituted styrene, e.g.,vinyl naphthalene, alpha alkyl substituted styrene (e.g., alpha methylstyrene) alkylene-substituted styrenes (particularlymonoalkyl-substituted styrenes such as vinyltoluene andethylvinylbenzene) and halo-substituted styrenes, such as bromo- orchlorostyrene and vinylbenzylchloride; and monovinylidene non-styrenicssuch as: esters of α,β-ethylenically unsaturated carboxylic acids,particularly acrylic or methacrylic acid, methyl methacrylate,isobornylmethacrylate, ethylacrylate, and butadiene, ethylene,propylene, acrylonitrile, and vinyl chloride; and mixtures of one ormore of said monomers. Preferred monovinylidene monomers includestyrene, acrylates and methacrylates. Examples of crosslinking monomers(i.e., polyvinylidene compounds) include polyvinylidene aromatics suchas divinylbenzene, divinyltoluene, divinylxylene, divinylnaphthalene,trivinylbenzene, divinyldiphenyl ether, divinyldiphenylsulfone, as wellas diverse alkylene diacrylates and alkylene dimethacrylates. Preferredcrosslinking monomers are divinylbenzene, trivinylbenzene, and ethyleneglycol dimethacrylate. The monomers used in each polymerization step maybe the same or different as those used in other steps, and may be usedin combinations.

The proportion of crosslinking monomer in the copolymer seed particlesis preferably sufficient to render the particles insoluble in subsequentpolymerization steps (and also on conversion to an ion-exchange resin),yet still allow for adequate imbibition of the phase-separating diluentand monomers of the second monomer mixture. In some embodiments, nocrosslinking monomer will be used. Generally, a suitable amount ofcrosslinking monomer in the seed particles is minor, i.e., desirablyfrom about 0.01 to about 5 molar percent, preferably from about 0.1 toabout 2.5 molar percent based on total moles of monomers in the seedparticles, with the balance comprising the first monovinylidene monomer.

Polymerization of the first monomer mixture may be conducted to a pointshort of substantially complete conversion of the monomers to copolymeror, alternatively, to substantially complete conversion. If incompleteconversion is desired, the resulting partially polymerized seedparticles advantageously contain a free-radical source therein capableof initiating further polymerization in subsequent polymerizationstages. The term “free-radical source” refers to the presence offree-radicals, a residual amount of free-radical initiator, or both,which is capable of inducing further polymerization of ethylenicallyunsaturated monomers. In such an embodiment of the invention, it ispreferable that from about 20 to about 95 weight percent of the firstmonomer mixture, based on weight of the monomers therein, be convertedto copolymer, and more preferably from about 50 to about 90 weightpercent. Due to the presence of the free radical source, the use of afree-radical initiator in a subsequent polymerization stage would beoptional. For embodiments where conversion of the first monomer mixtureis substantially complete, it may be necessary to use a free-radicalinitiator in subsequent polymerization stages.

The free-radical initiator may be any one or a combination ofconventional initiators for generating free radicals in thepolymerization of ethylenically unsaturated monomers. Representativeinitiators are UV radiation and chemical initiators, such asazo-compounds like azobisisobutyronitrile; and peroxygen compounds suchas benzoyl peroxide, t-butylperoctoate, t-butylperbenzoate andisopropylpercarbonate. Other suitable initiators are mentioned in U.S.Pat. Nos. 4,192,921; 4,246,386; and 4,283,499. The free-radicalinitiators are employed in amounts sufficient to induce polymerizationof the monomers in a particular monomer mixture. The amount will vary,as those skilled in the art can appreciate, and will depend generally onthe type of initiators employed, as well as the type and proportion ofmonomers being polymerized. Generally, an amount of from about 0.02 toabout 2 weight percent is adequate, based on total weight of the monomermixture.

The first monomer mixture used to prepare the seed particles isadvantageously suspended within an agitated suspending medium comprisinga liquid that is substantially immiscible with the monomers, preferablywater. Generally, the suspending medium is employed in an amount fromabout 30 to about 70, and preferably from about 35 to about 50 weightpercent based on total weight of the monomer mixture and suspendingmedium. Various suspending agents are conventionally employed to assistwith maintaining a relatively uniform suspension of monomer dropletswithin the suspending medium. Illustrative suspending agents aregelatin, polyvinyl alcohol, magnesium hydroxide, hydroxyethylcellulose,methylcelluloses, and carboxymethylmethylcellulose. Other suitablesuspending agents are disclosed in U.S. Pat. No. 4,419,245. The amountof suspending agent used can vary widely depending on the monomers andsuspending agents employed.

The seed particles may be of any convenient size. In general, the seedparticles desirably have a volume average particle diameter of fromabout 75 to about 1000 microns, preferably from about 150 to about 800microns, and more preferably from about 200 to about 600 microns. Thedistribution of the particle diameters may be gaussian or uniform (e.g.,90 percent of the particles having diameters within +/−100 microns ofthe median particle diameter size). Volume average particle diameter maybe determined by a commercially available instrument designed to makesuch measurement, such as a Criterion Model PC-320 Particle SizeAnalyzer available from the HIAC-Royco Company. Volume average particlediameter may also be determined by screen analysis, such as ASTMD-2187-74, using appropriate screen sizes.

Copolymer beads may be prepared by providing a plurality of the seedparticles and, thereafter, adding the second monomer mixture such thatthe mixture is imbibed by the seed particles and polymerizationconducted therein. This step is preferably conducted as a batch-seededprocess or as an insitu batch-seeded process, as described hereinafter.The second monomer mixture may also be added intermittently orcontinuously under polymerizing conditions, such as in U.S. Pat. No.4,564,644 previously discussed.

In the so-called “batch-seeded”process, seed particles comprising fromabout 10 to about 40 weight percent of the porous copolymer bead productare preferably suspended within a continuous suspending medium. A secondmonomer mixture containing a free radical initiator is then added to thesuspended seed particles, imbibed thereby, and then polymerized.Although less preferred, the seed particles can be imbibed with thesecond monomer mixture prior to being suspended in the continuoussuspending medium. The second monomer mixture may be added in one amountor in stages. The second monomer mixture is preferably imbibed by theseed particles under conditions such that substantially nopolymerization occurs until the mixture is substantially fully imbibedby the seed particles. The time required to substantially imbibe themonomers will vary depending on the copolymer seed composition and themonomers imbibed therein. However, the extent of imbibition cangenerally be determined by microscopic examination of the seedparticles. The second monomer mixture desirably contains from about 3 toabout 25, preferably from about 5 to about 20 weight percent ofcrosslinking monomer based on total weight of monomers in the secondmonomer mixture, with the balance being the second monovinylidenemonomer.

In an insitu batch-seeded process, seed particles comprising from about10 to about 80 weight percent of the porous copolymer bead product areinitially formed by suspension polymerization of the first monomermixture. The gel seed particles can have a free-radical source thereinas previously described, which is capable of initiating furtherpolymerization. Optionally, a polymerization initiator can be added withthe second monomer mixture where the seed particles do not contain anadequate free radical source or where additional initiator is desired.In this embodiment, seed preparation and subsequent polymerizationstages are conducted insitu within a single reactor. A second monomermixture is then added to the suspended seed particles, imbibed thereby,and polymerized. The second monomer mixture may be added underpolymerizing conditions, but is preferably added to the suspendingmedium under conditions such that substantially no polymerization occursuntil the mixture is substantially fully imbibed by the seed particles.The composition of the second monomer mixture corresponds to thedescription previously given for the batch-seeded embodiment.

Conditions employed to polymerize ethylenically unsaturated monomers arewell known in the art. Generally, the monomers are maintained at atemperature of from about 50° C. to about 90° C. for a time sufficientto obtain a desired degree of conversion. Typically, an intermediatetemperature of from about 60° C. to about 80° C. is maintained untilconversion of monomer to copolymer is substantially complete and,thereafter, the temperature is increased to complete the reaction. Theresulting porous copolymer beads may be recovered from the suspendingmedium by conventional methods.

In general, strong acid resins are prepared by reacting the copolymerwith a sulfonating agent such as sulfuric acid, chlorosulfonic acid, orsulfur trioxide. Contact with the sulfonating agent can be conductedneat or with a swelling agent. Contact is typically conducted attemperatures from about 0° C. to about 150° C.

As indicated, the sulfonated cation exchange resins of the presentinvention are particularly suited for use in water treatment modules fortreating power plant condensate water. In use, the subject resins areplaced within an ion exchange bed. A water intake directs untreatedwater from the power plant condensate water loop into the bed where itcontacts the subject resin. Once treated, the water exits the bed by wayof a water outtake where the treated water continues through the waterloop and is recycled. Although not required, the subject cation exchangeresins are preferably used in combination with an anion resin. Thesesystems are optimized (operating pressures, flow rates, etc.) foroperating conditions of a given power plant.

SPECIFIC EMBODIMENTS OF THE INVENTION

The following specific examples illustrate the invention and should notbe construed as limiting the scope of the appended claims. The “seeded”copolymers described below were prepared using common suspensionpolymerization techniques as described in U.S. Pat. Nos. 4,564,644;5,231,115 and Re, 34,112, each of which are incorporated herein in theirentirety. Polymerization was conducted in a computer-automated 1-gallonstainless steel reactor that was jacketed for heating. The copolymerreactor was equipped with an agitator, a feed port, a drain valve, avent valve, a nitrogen line, and a frangible. In Example I, anadditional feed line was used for continuous addition (i.e., “con-add”)of monomer from a con-add tank into the reactor as described below.

EXAMPLE 1

Seed particles were made by preparing an aqueous phase composed ofapproximately 840 grams (g) of deionized water with 2.5 g of a 67percent aqueous solution of dichromate solution and 360 g of a 1 weightpercent solution of carboxy methyl methyl cellulose (CMMC) aqueoussolution used as a suspension aide. An organic phase was addedconsisting of a crosslinking agent, divinylbenzene (DVB); amonovinylidene monomer, (styrene or isobornylmethacrylate (IBMA)); andan initiator (t-butylperoctoate and t-butylperbenzoate). The specificquantities of crosslinking agent and monovinylidene monomer used areindicated below in Table 1. Once the constituents were combined, thereactor was stirred at 25° C. for 60 minutes at 300 RPM to properly sizethe monomer droplets. The temperature was then ramped at 0.4° C./min to70° C. and held at temperature for 860 minutes at 150 RPM. As afinishing step, a final ramp of 0.4° C./min to 110° C. was held for 2hours. The reactor was than cooled to 25° C. and the copolymer seedparticles were removed, washed, de-watered, air-dried and screened.

TABLE 1 Sample A B C D E F Wt % Crosslinking Agent 0.30 0.24 0.75 0.501.18 2.45 in Monomer Mixture Molar % Crosslinking 0.51 0.41 0.60 0.412.00 2.00 Agent in Monomer Mixture Organic Phase Styrene (g) 0 0 11911194 0 1170 IBMA (g) 1195.94 1160 0 0 1160 0 DBC (55 wt %) (g) 6.56 5.0916.4 10.9 25.5 54.5

The seed particles were subsequently imbibed with a second polymercomponent according to the following procedure: approximately 300 g ofdried and screened seed particles were added to an aqueous phaseconsisting of 775 g of deionized water, 20 g of a 1 weight percentsodium lauryl sulfate (SLS) aqueous solution and 3.3 g of a 67 weightpercent aqueous dichromate solution. An imbibe monomer mixture ofstyrene and DVB were added while stirring to swell the copolymer seed.The imbibe phase was added over 15 minutes through the top port on thereactor and allowed to mix for 60 minutes at 230 RPM. After the 60minute imbibe time a gelatin shot consisting of 250 g of hot deionizedwater, 20 g of 1 weight percent aqueous solution of SLS and 3.2 g ofgelatin were added through the top port. The reactor was then sealed,ramped at 0.4° C./min to 78° C. and held for 600 minutes at 270 RPM.

A third polymer component was then added as a “con-add” portion. Morespecifically, styrene and DVB were added to the imbibed seeds over 200to 300 minutes while the reactor was at 78° C. After the reaction hadspent 600 minutes at 78° C. it was followed by a second ramp at 0.4°C./min to 110° C. where it was held for two hours. The reactor was thencooled to 25° C. and the copolymer was removed, washed, de-watered,air-dried and screened.

Table 2 provides the specific quantities of crosslinking agent andmonovinylidene monomer that were used for each copolymer.

TABLE 2 Sample A B C D E F Molar % Crosslinking 0.5/6.6/2.4 0.4/5.8/0.6/4.9/2.4 0.4/5.8/2.4 2/4.1/ 2/4.1/2.4 Agent in: S/IM/CA 2.4 2.4Weight ratio of 1/1.24/1.43 1/1.24/ 1/1.24/1.43 1/1.24/2.86 1/0.6/1/0.6/2.66 S/IM/CA 2.86 2.66 Seed Component (S): Seed Type AcrylicAcrylic Styrenic Styrenic Acrylic Styrenic Weight % 0.3 0.2 0.8 0.5 1.22.5 Crosslink Agent Molar % 0.5 0.4 0.6 0.4 2.0 2.0 Crosslink AgentImbibe Component (IM) in grams: Styrene 320.5 324.7 349.7 346.0 163.8171.0 DVB (55 wt %) 54.5 47.4 40.6 47.3 16.2 16.4 t-butylperoctoate 0.540.54 0.54 0.54 0.54 0.54 t-butylperbenzoate 0.36 0.36 0.36 0.36 0.360.36 Con-Add Component (CA) in grams: Styrene 416.1 832.0 416.1 832.0774.1 774.0 DVB (55 wt %) 23.4 46.8 23.4 46.8 43.5 43.5 wherein “S”designates the seed polymer component, “IM” designates the imbibedpolymer component, “CA” designates the continuous addition polymercomponent. Samples C, D, and F do not form part of the present inventionbut are included for purposes of comparison.

The resins were then sulfonated in standard laboratory 5-literthree-neck, glass, round-bottom reactors. Each of these reactors wereequipped with a glass shaft and Teflon™ paddle and two infrared heatinglamps. 330 Grams of copolymer, 3300 g of 98 weight percent sulfuric acidand 330 g of methylene chloride. The methylene chloride addition wasmade over 15 minutes while the acid-copolymer slurry was being stirred.The temperature was ramped at 1° C./min to 115° C. and held for 2 hoursbefore cooling. The acid-resin slurry was then slowly diluted byportions lower concentration sulfuric acid and then washed in water.

Several of the sulfonated resins were then treated with an antioxidant,2,6-di-t-butyl-α-dimethylamino-p-cresol using the following procedure: 1liter of sulfonated resin was added to a 1 liter solution of deionizedwater containing 10 ml 1N HCl. With constant mixing, a solution of 2.1 gof 2,6-di-t-butyl-α-dimethylamino-p-cresol in 150 ml deionized water and25 ml 1N HCl was added to the resin mixture over a 30 minute period. Theresin mixture was stirred for an additional 30 minutes, followed by arinse with deionized water.

The prepared cation exchange resins were subjected to an acceleratedoxidation test after which the molecular weight distribution of theleachables released by the resin was measured by gel permeationchromatography. As previously described, several examples included mixedresins wherein an anion exchange resin was mixed with the subject cationexchange resin. None of the cation exchange resins in the mixed resinswere treated with the aforementioned antioxidant. The results of thetesting is provided in Table 3 below.

The accelerated resin oxidation procedure for the cation exchange resinscomprised the following: 125 ml of the cation exchange resin was rinsedwith 1.0 liter deionized water, then the resin and 625 ml deionizedwater were placed into a one liter flask with a condenser, stir paddleand glass bubbler. The resin was stirred at 60 rpm and heated at 80° C.for 14 days with a 20 cc/min flow of oxygen bubbling through themixture. The mixture was cooled, the solution separated from the resin,and the molecular weight distribution of the leachables was measured.

The accelerated resin oxidation procedure for the mixed anion and cationexchange resins comprised the following: 75 ml of the cation exchangeresin and 75 ml of the anion exchange resin (DOWEX™ MONOSPHERE™ 550A-OH)were individually rinsed with 1.0 liter deionized water. Both resins and450 ml deionized water were placed into a one liter flask with acondenser, stir paddle and glass bubbler. The resin was stirred at 120rpm and heated at 80° C. for 7 days with a 20 cc/min flow of oxygenbubbling through the mixture. The mixture was cooled, the solutionseparated from the resin, and the molecular weight distribution of theleachables was measured.

Leachable analysis for each sample resin was conducted by way of gelpermeation chromatography under the following conditions: Mobile phase,0.05 M Na₂SO₄ with 1.00 ml 1 N NaOH per liter and pH adjusted to 8.0with 5 percent H₃PO₄ at a flow rate of 0.40 ml/min; 25 microlitersinjection volume and detector set at 229 nm. calibration standards,sulfonated polystyrene (Na salt) from American Polymer Standards withweight average molecular weights (Mw)=41,000; 31,000; 17000; 6,500;4,800. The average molecular weight for the leachable solutions werecalculated for species with weights between 1,000 and 100,000 daltons.The specific equipment utilized included: Waters Assoc. liquidchromatography system composed of a 600E pump/controller, 712 WISPautosampler, 486 absorbance detector, Millennium 2.00 analysis software.Synchropak GPC100 (50×4.6 mm), GPC300 (250×4.6mm), GPC100 (250×4.6 mm)columns in series.

Crud removal analysis was conducted pursuant to the procedure describedin: T. Izumi, et. al., “Crud Removal Characteristics of Newly DevelopedIon Exchange Resins (2^(nd) Report)”, Proc. 52^(nd) Int. Water Conf.,1991, p. ⁴54.

TABLE 3 Molar % Molar Ratio of Molar Ratio of Avg. Mw of SeedCrosslinking Crosslink Agent Crosslink Agent Leachable Sample Type agent(S/IM) (S/CA) (daltons) A Acrylic 0.5/6.6/2.4 0.076 0.21  5,000 CStyrenic 0.6/4.9/2.4 0.12 0.25 10,300 B Acrylic 0.4/5.8/2.4 0.069 0.17 3,300 D Styrenic 0.4/5.8/2.4 0.069 0.17 22,000  A′ Acrylic 0.5/6.6/2.40.076 0.21  7,390  C′ Styrenic 0.6/4.9/2.4 0.12 0.25 16,800 wherein “S”,“IM” and “CA” are as defined above and samples A′ and C′ are mixed resinbed including the same cation exchange resin as provided in samples Aand C, respectively. Sample C′ is not part of the present invention butis included for purposes of comparison.

The data provided in Table 3 shows that the replacement of the styrenicmonomer in the low crosslinked seed portion of the copolymer with anon-styrenic (acrylic) monomer resulted in a significant reduction inthe average molecular weight of the leachables released by the cationexchange resin—see for example Sample A vs. C,B vs. D, and A′ vs. C′.

EXAMPLE 2

Seed particles were prepared by the procedure given in Example 1 usingeither isobornylmethacrylate or methylmethacrylate (MMA) as themonovinylidene monomer. The specific quantities of crosslinking agentand monovinylidene monomer used are indicated below in Table 4.

TABLE 4 Sample G H I Wt % Crosslinking Agent 0.25 1.18 0.25 in MonomerMixture Molar % Crosslinking 0.42 2.0 0.19 Agent in Monomer MixtureOrganic Phase MMA (g) 0 0 1197 IBMA (g) 1197 1174 0 DVB (55 wt %) (g)5.45 25.7 5.45

The seed particles were subsequently imbibed with a second polymercomponent according to the following procedure: approximately 240 g ofdried and screened seed particles were added to an aqueous phaseconsisting of 775 g of deionized water, 20 g of a 1 weight percentsodium lauryl sulfate (SLS) aqueous solution and 3.3 g of a 67 weightpercent aqueous dichromate solution. An imbibe monomer mixture ofstyrene and DVB were added while stirring to swell the copolymer seed.The imbibe phase was added over 15 minutes through the top port on thereactor and allowed to mix for 60 minutes at 230 RPM. After the 60minute imbibe time a gelatin shot consisting of 250 g of hot deionizedwater, 20 g of 1 weight percent aqueous solution of SLS and 3.2 g ofgelatin were added through the top port. The reactor was then sealed,ramped at 0.4° C./min to 78° C. and held for 600 minutes at 270 RPM.Then the temperature was increased at 0.4° C./min to 110° C. where itwas held for two hours. The reactor was then cooled to 25° C. and thecopolymer was removed, washed, de-watered, air-dried and screened. Table5 provides the specific quantities of crosslinking agent andmonovinylidene monomer that were used for each copolymer.

TABLE 5 Sample G H I Molar % Crosslinking Agent in S/IM 0.4/4.7 2/7.40.2/8.3 Weight ratio of S/IM   1/4.0   1/4.0   1/3.5 Seed Component (S):Acrylate Seed Type IBMA IBMA MMA Weight % Crosslink Agent 5.7 8.9 10.0Molar % Crosslink Agent 4.7 7.4 8.3 Imbibe Component (IM) in grams:Styrene 902.4 902.4 691.2 DVB (55 wt %) 104.7 174.5 153.6t-butylperoctoate 0.54 0.54 0.54 t-butylperbenzoate 0.36 0.36 0.36 MolarCrosslink Ratio (S/IM) 0.09 0.27 0.02 Avg. Mw of Leachable (daltons)5,000 1,400 2,800 wherein “S” designates the seed polymer component,“IM” designates the imbibed polymer component.

The copolymers were sulfonated using the procedures described inExample 1. The sulfonated resins were then treated with an antioxidant,2,6-di-t-butyl-α-dimethylamino-p-cresol using the procedure described inExample 1.

The prepared cation exchange resins were subjected to an acceleratedoxidation test after which the molecular weight distribution of theleachables released by the resin was measured by gel permeationchromatography. The procedures used are described in Example 1. Theresults of the testing are provided above in Table 5.

The average molecular weight for leachables from styrenic resins withcrosslink levels of 0.2 percent, 0.4 percent and 2.0 percent areexpected to be about 60,000, 30,000 and 6,000, respectively. The dataprovided in Table 5 shows that the replacement of the styrenic monomerin the low crosslinked seed portion of the copolymer with a non-styrenic(acrylic) monomer resulted in a significant reduction in the averagemolecular weight of the leachables released by the cation exchangeresin.

What is claimed is:
 1. A gel-type copolymer bead having aninterpenetrating polymer network of multiple polymer componentscomprising: a first polymer component derived from a first monomermixture comprising: a first monovinylidene monomer having a styreniccontent less than about 50 molar percent and an optional firstcrosslinking agent; a second polymer component derived from a secondmonomer mixture comprising: a second monovinylidene monomer having astyrenic content greater than about 50 molar percent and a secondcrosslinking agent; and wherein the ratio of the molar percentcrosslinking agent of the first polymer component to the second polymercomponent is less than about 0.7.
 2. The copolymer bead of claim 1wherein the ratio of the molar percent crosslinking agent of the firstpolymer component to the second polymer component is less than about0.4.
 3. The copolymer bead of claim 2 wherein the ratio of the molarpercent crosslinking agent of the first polymer component to the secondpolymer component is less than about 0.1.
 4. The copolymer bead of claim1 wherein the first monovinylidene monomer has a styrenic content lessthan about 10 molar percent and the first monomer mixture comprises lessthan about 5 molar percent of a crosslinking agent.
 5. The copolymerbead of claim 4 wherein the first monovinylidene monomer mixturecomprises less than about 1 molar percent crosslinking agent.
 6. Thecopolymer bead of claim 4 wherein the second monovinylidene monomer hasa styrenic content of greater than about 90 molar percent.
 7. Thecopolymer bead of claim 1 wherein the first monovinylidene monomercomprises a non-styrenic monomer including at least one of the followingfunctional groups: acrylate, methacrylate, butadiene, ethylene,propylene, acrylonitrile, and vinyl chloride.
 8. The copolymer bead ofclaim 7 wherein the first monovinylidene monomer includes at least oneof an acrylate and methacrylate.
 9. The copolymer bead of claim 8wherein the first monovinylidene monomer includes isobornylmethacrylate.10. The copolymer bead of claim 1 wherein the bead is sulfonated to forma sulfonated ion exchange resin.
 11. A process for making gel-typecopolymer beads comprising: providing gel-type seed particles preparedby polymerizing a first monomer mixture comprising: a firstmonovinylidene monomer having a styrenic content less than about 50molar percent and an optional first crosslinking agent; contacting theseed particles with a second monomer mixture comprising: a secondmonovinylidene monomer having a styrenic content greater than about 50molar percent and a second crosslinking agent wherein the ratio of themolar percent crosslinking agent of the first monomer mixture to thesecond monomer mixture is less than about 0.7, and polymerizing thesecond monomer mixture to form an interpenetrating polymer network withthe seed particles.
 12. The process of claim 11 wherein the step ofcontacting the seed particles with a second monomer mixture comprisesimbibing the seed particles with the second monomer mixture andsubsequently polymerizing the second monomer mixture to form aninterpenetrating polymer network.
 13. The process of claim 12 whereinthe imbibed seed particles are contacted with a continuous addition of amonomer mixture which is polymerized to form an interpenetrating polymernetwork.
 14. The process of claim 11 wherein the step of contacting theseed particles with the second monomer mixture comprises continuouslyadding and polymerizing the second monomer mixture to form aninterpenetrating polymer network.
 15. The process of claim 11 wherein:the first monomer mixture has a styrenic content of less than about 10molar percent and less than about 5 molar percent of the firstcrosslinking agent, the second monomer mixture has a styrenic content ofgreater than about 90 molar percent; and the ratio of the molar percentcrosslinking agent of the first monomer mixture to the second monomermixture is less than about 0.1.